1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to an apparatus and method for transmitting power headroom information in a multi-carrier system.
2. Discussion of the Related Art
In general, a wireless communication system uses one bandwidth for data transmission. For example, a 2nd generation wireless communication system uses a bandwidth of 200KHz˜1.25MHz, and a 3rd generation wireless communication system uses a bandwidth of 5MHz˜10 MHz. To support growing transmission capacity, the 3rd generation partnership project (3GPP) long term evolution (LTE) or the institute of electrical and electronics engineers (IEEE) 802.16m has recently expanded its bandwidth to 20MHz or higher. It may be necessary to increase the bandwidth to obtain high transmission capacity. However, if a large bandwidth is supported even when a service request level is low, it may cause significant power consumption.
Therefore, a carrier having one bandwidth and a center frequency is defined, and a multiple component carrier system (hereinafter, referred to as a multi-carrier system) capable of transmitting and/or receiving broadband data by using a plurality of carriers has been introduced. By using one or more carriers, a narrowband and a broadband are both supported. For example, if one carrier corresponds to a bandwidth of 5MHz, four carriers are used to support a bandwidth of up to 20MHz. As such, in a multi-carrier system supporting a plurality of carriers, each carrier is referred to as a component carrier (CC).
Meanwhile, as one method of effectively scheduling a user equipment (UE) by a base station (BS), power headroom (PH) information of the UE is used. A PH implies a difference between a maximum transmission power assigned to the UE and a transmission power estimated by the UE, that is, implies an extra power of the UE. The power headroom information (PHI) is essential information for effectively allocating uplink resources in wireless communication and for reducing battery consumption of the UE. When the UE provides the PHI to the BS, the BS can estimate a level of an uplink maximum transmission power that can be handled by the UE. Therefore, the BS can perform uplink scheduling in a range not beyond a limit of the estimated uplink maximum transmission power. When the UE transmits the PHI to the BS, it is called power headroom reporting (PHR).
However, since the conventional PHR provides only a PH for each carrier in case of the multi-carrier system, the BS cannot exactly know a maximum transmission power value which is used as a reference for calculation of the PH. Therefore, uplink scheduling of the BS is uncertain in some parts, and in particular, uplink transmission power scheduling depending on the PHI may cause a problem in a system (i.e., a multiple component carrier system) in which a PH for each CC needs to be individually and integrally taken into account.